1. Field of the Invention
The present invention relates to a bobbin for making windings on a core for use in a switching power supply driven with a high frequency.
2. Description of the Related Art
A switching power supply is used as a power supply which is high in efficiency and which can be made small in size. In order to satisfy safety standards such as UL, CSA, IEC and so on, a predetermined insulation countermeasure is given to such a power supply. Specifically, other than use of a winding bobbin of an insulating material, an insulating material is inserted within such a bobbin.
FIG. 20 is a sectional view illustrating an example of a conventional high-frequency transformer.
A bobbin 2 is provided so as to be buried in a core 1, and a primary winding 3 is wound on the inner circumference of the bobbin 2 over a half of its depth from its bottom, with barrier tapes 4 interposed between the bobbin 2 and each f the opposite sides of the primary winding 3. After the primary winding 3 is wound by the required number of turns, an insulating tape 5 is provided over the surface of the primary winding 3. Then, a secondary winding 6 is wound on the insulating tape 5, with barrier tapes 7 interposed between the bobbin 2 and each of the opposite sides of the secondary winding 6, and an insulating tape 8 is further wound over the surface of the secondary winding 6. Here, each of the barrier tapes 4 and 7 is provided so as to have a thickness in a range of from 2 to 3 mm. Thus, by the provision of the barrier tapes 4 and 7, insulation distances between the primary and secondary windings and between the core and each of the primary and secondary windings are set so as to satisfy the safety standards. Then, the surface of the windings and leading-out wires are covered with insulating tubes (not-shown).
There is no problem in the case where the length L of the bobbin over which winding is provided is sufficiently long. If the length L is short, however, the performance of the transformer is lowered. For example, consideration will be made upon PQ 50.50 (for example, the size with which an output of about 1 KW can be extracted under the switching frequency of 100 KHz) which is the largest of the cores available in the market at present. Although the winding width of this bobbin is 32 mm, the width over which winding can be made becomes 26 mm when the width of each of the opposite side barrier insulating tapes is set to 3 mm (that is, the total width is set to 6 mm taking scattering into consideration, though it does not matter in the case where the width of each side barrier insulating tape is set to 2 mm, and hence the total width is set to 4 mm, in accordance with the UL standards). Accordingly, the total sectional area of the winding becomes about 4/5 of the bobbin space. In such a case, it is possible to obtain a winding having the same winding resistance and 4/5 inductance if the number of turns is set to 4/5 square root, and the sectional area of wire material to be used is set to 4/5 square root. Therefore, even if a safety standard countermeasure is performed by increasing the switching frequency to be 5/4-fold, it is possible to produce a transformer with almost the same copper loss and iron loss as those in the case of using the whole of the bobbin space.
However, it is usually difficult to provide such a performance as mentioned above in the case of an output in a range of from 50 to 300 watt often used in office automation equipment. For example, in the case of "EI30" with which it is possible to obtain an output of about 150 W at 100 KHz, the bobbin length is 13 mm and the width over which winding can be made is 7 mm if 3 mm-thick barrier insulating tapes are wound on the opposite sides of the winding, so that the axial length of a coil (hereinafter referred to as "coil length") becomes extremely short.
As a result, since the winding structure becomes short in its coil length and large in its winding thickness, not only can enough of a coil sectional area not be obtained but also the magnetic flux leakage of the transformer becomes large so that copper loss becomes large and spike voltage becomes high. Thus, the shape of the winding structure is not suitable for a switching power supply.
Further, in the case of "EI28" with which an output of about 150 W can be obtained by making the switching frequency high to 500 KHz, the bobbin length is about 9.6 mm, and the winding length becomes 3.6 mm if barrier insulating tapes are wound on the winding. Thus, it is almost impossible to realize a transformer.
In order to improve such a state even slightly, cores in which the length in the direction of the center pole axis is elongated without changing any other size have been increased recently. Although this improvement can make the sectional area of coil larger, it makes the effective magnetic flux sectional area smaller and makes the core loss larger at the same time, so that it cannot be a fundamental solution. At the present time, respective elements, ICs, and other parts have been improved in order to reduce the size of an apparatus, and also as for core material, cores corresponding to 200 KHz, 500 KHz, 1 MHz and so on have been realized.
In the above-mentioned conventional technique, however, there is a problem of design in the size and shape of a core because of limitations due to the safety standards, independently of the advance of core materials. This is a large obstacle in making a transformer small in size and high in frequency.
FIG. 21 is a conventional example of a pot core 17. In this conventional example, a winding is divided into a plurality of portions in the axial direction of a spool bobbin 18. That is, a primary winding 19 and a secondary winding 20 wound on the spool bobbin 18 side by side in the axial direction of the spool bobbin 18. The bobbin 18 having the primary and secondary windings 19 and 20 wound thereon is fixedly accommodated in the pot core 17. In this configuration, however, the degree of coupling is poor since the primary and secondary windings 19 and 20 are separated from each other to be upper and lower parts respectively,
FIG. 22 is a conventional example of an EE-type core, in which a bobbin is constituted by a rectangular hollow primary winding bobbin 21 and a rectangular hollow secondary winding bobbin 22 coupled with the lower portion of the bobbin 21. A plurality of pins 23 are provided at predetermined intervals so as to project from the bottom of the secondary winding bobbin 22. A primary winding 24 is wound on the primary winding bobbin 21, and a secondary winding 25 is wound on the secondary winding bobbin 22. The center leg portion of an E-shaped core 26 is inserted into the hollow portion of the primary winding bobbin 21, and the center leg portion of the other E-shaped core (not-shown) is inserted into the hollow portion of the secondary winding bobbin 22 to thereby form a transformer. In such a bobbin structure, coupling is poor because of a gap produced between the primary and secondary windings. Further, windings are exposed so that it is difficult to ensure a sufficient creepage distance or a sufficient insulation distance and it is therefore difficult to cope with the safety standards in the case of a high-frequency core of the type in which the whole surface of the windings are covered with the core.